A large number of physiological processes are controlled by the endogenous cannabinoids. Most of these effects have been attributed to action at either the cannabinoid CB1 or CB2 receptors. Yet there are effects that clearly are not CB1- or CB2-mediated which may not be receptor mediated, but there is also sufficient evidence to suggest the involvement of the vanilloid receptor (TRPV1) in cannabinoid effects and at least two other cannabinoid receptor subtypes defined only pharmacologically until now. Very recently, two orphan G-protein coupled receptors (GPCRs), GPR35 and GPR55, have been suggested to be cannabinoid receptors, with a fairly wide range of cannabinoid ligands reported to display affinity/efficacy at each. We have cloned and expressed both GPR35 and GPR55 and our preliminary studies confirm that they are activated by multiple cannabinoid compounds. In work proposed here, we plan to characterize these two GPCRs through ligand binding and functional studies. Recently developed computer models of GPR35 and GPR55 in their inactive and activated states will be used to guide mutation studies of each receptor. These models are informed by our extensive modeling and mutation experience with the cannabinoid CB1 and CB2 receptors. The goal of mutation studies will be not only to identify residues involved in ligand recognition, but also those residues important for receptor activation. In GPR35 and GPR55 ligand recognition studies, the involvement of specific amino acids with specific ligand functional groups will be tested using carefully chosen compounds. Results of mutation studies in vitro will be used to refine computer receptor models, such that at any given time, these models reflect the current state of knowledge in the field. Because work thus far on GPR35 and GPR55 has not identified a high affinity antagonist for either receptor and because such antagonists would be very valuable tools for studying these two receptors, another goal of the proposed work will be to design antagonists for each receptor sub-type. These compounds will be designed at UNCG (Reggio), synthesized at Research Triangle Institute (Seltzman), and evaluated at CPMCRI (Abood). High affinity antagonists that emerge from this work will be radiolabeled and made available to the scientific community. Determining the distribution of GPR35 and signal transduction pathways of GPR35 and GPR55 will help define their physiological and pathophysiological roles.